1. Field of Invention
The present invention relates in general to nuclear well logging, and pertains in particular to improved methods and apparatus for spectroscopic analysis of capture gamma ray energy spectra to provide information regarding the lithology of earth formations surrounding a well borehole.
2. The Prior Art
Various techniques have been utilized in the past to irradiate a geological formation with neutrons and to detect and process the resultant gamma ray energy spectra for borehole elemental constituent analysis. At its most fundamental level, such an analysis of capture gamma spectra reveals information concerning the presence of hydrogen, silicon, calcium, chlorine, sulfur and iron.
The derivation of such information depends upon an accurate constituent analysis of the formation gamma ray spectra. An important and basic technique for performing such an analysis is disclosed in U.S. Pat. No. 3,521,064, issued on July 21, 1970, to Moran, et al. In accordance with the Moran et al. teaching, a detected gamma ray energy spectrum derived from a formation of unknown composition by a spectroscopy logging tool is compared with a composite spectrum made up of weighted laboratory-derived, standard spectra of the constituents postulated to comprise the formation. The weight coefficients for the standard spectra which give the best fit of the composite spectrum to the unknown spectrum, as determined, for example, by the method of least squares, represent the relative proportions of the constituents in the formation. By appropriate selection and weighting of the standard spectra, the proportions of the constituents of interest may be obtained, from which the desired information regarding oil content may be derived.
An improvement over the Moran et al. technique is described in U.S. Pat. No. 4,055,763 issued to Stephen Antkiw on Oct. 25, 1977 and assigned to the assignee of the present invention, in which the spectroscopic technique is improved by providing a method and means for optimally gating the gamma ray detection periods relative to the neutron pulses in accordance with measured decay times of the thermal neutrons in the formation. In addition, that patent discloses a technique and means for controlling both the duration and repetition rate of the neutron pulses as a function of the measured decay times to provide an overall optimized decay time-spectroscopy operating cycle. Furthermore, that patent proposes that qualitative indicators of formation lithology, salinity, porosity and shaliness may be derived by taking the ratios of various combinations of the relative elemental contributions to the capture gamma ray spectra. Finally, knowledge of the thermal neutron decay time permits the determination of the formation macroscopic neutron absorption cross-section, .SIGMA..
A further improvement to the spectroscopic logging techniques advanced in the Moran and Antkiw patents is described in copending U.S. Patent Application Ser. No. 187,123 by Grau, et al., now U.S. Pat. No. 4,394,574, entitled "Methods and Apparatus for Constituent Analysis of Earth Formations" in which a technique is advanced by which the spectroscopic analysis is substantially improved through the convolution of the elemental standard spectra in order to account for the degrading effects on detector resolution resulting from the high temperature borehole logging environment.
These prior patents and application illustrate the basic principles involved in determining the relative percent contributions to the detected spectra due to the presence of certain elements in the subsurface geological formation. While this information is of great interest and of high value, it would also be of interest to determine information directly related to the relative volume fractions of commonly encountered basic formation components such as limestone, sandstone, porosity, dolomite, etc. which contain one or more of the elements measured by the spectroscopic techniques proposed by the above discussed patents.
U.S. Pat. Nos. 3,928,763; 3,930,153; and 3,930,154, all issued to H. D. Scott, utilize a weighted least squares spectral fitting technique similar to that described in the Moran patent for obtaining relative elemental yields of elements contributing to a spectrum. Information bearing on the macroscopic neutron absorption cross section is then used in a processing technique which derives further information relating to the volume fractions of the basic formation components such as sandstone, limestone, etc.
While the techniques of the Scott patents are useful, they depend on rather complicated calculations necessitated by the various corrections and compensations necessary to produce a valid result. One such compensation is embodied in a function dependent on formation macroscopic neutron absorption cross section, called the "Compensating Function" (f(.SIGMA.)), which compensates for the time dependence of the actual gamma ray count upon the neutron capture cross section but which fails to take into account the spatial dependence of the neutron flux distribution which can be expected to vary from formation to formation in proportion to, among other things, the porosity of the formation and the neutron absorption cross section of the formation itself. In other words, the Scott "Compensating Function" might be expected to be correct only for formations of constant makeup and constant porosity.
Additionally, the Scott technique of determining formation component volume fractions requires detailed knowledge of the microscopic neutron capture cross sections for the various elements expected to be encountered, as well as the absolute values of so-called "calibration constants", (Gi) defined by source-detector configuration, the neutron source strength, and the portion of the gamma-ray spectrum to be analyzed. Therefore, a clear and straightforward technique which is not prone to the lengthy computations and accompanying uncertainties inherent in the technique of the Scott patents is desired.